The invention relates to an elastic fiber including at least one and preferably two polyolefin thermoplastic components.
Ethylenexe2x80x94propylene copolymers and blends of isotactic polypropylene and ethylene propylene rubber are well known in the prior art. However, the traditional Ziegler-Natta catalysts used to make the ethylene propylene elastomer have limitations. Thus polymers which are simultaneously uniform in compositional distribution, have substantially stereospecific propylene residues and have less than 35 wt. % ethylene are not available with these catalysts. These limitations in the synthesis have lead to the absence of elastic fibers from blends of ethylene propylene copolymers and isotactic polypropylene.
U.S. Pat. No. 3,882,197 suggests blends of stereoregular propylene/alpha-olefin copolymers, stereoregular propylene, and ethylene copolymer rubbers.
U.S. Pat. No. 3,888,949 suggests the synthesis of blend compositions containing isotactic polypropylene and copolymers of propylene and an alpha-olefin, containing between 6-20 carbon atoms, which have improved elongation and tensile strength over either the copolymer or isotactic polypropylene. Copolymers of propylene and alpha-olefin are described wherein the alpha-olefin is hexene, octene or dodecene. However, the copolymer is made with a heterogeneous titanium catalyst resulting in copolymers with non-uniform composition distribution and a broad molecular weight distribution. Non-uniform intramolecular compositional distribution is evident in U.S. Pat. No. 3,888,949 by the use of the term xe2x80x9cblockxe2x80x9d in the description of the polymer where the copolymer is described as having xe2x80x9csequences of different alpha-olefin content.xe2x80x9d Within the context of the invention described above the term sequences describes a number of olefin monomer residues linked together by chemical formed during polymerization.
U.S. Pat. No. 4,461,872, improved on the properties of the blends described in U.S. Pat. No. 3,888,949 by using another heterogeneous catalyst system which is expected to form copolymers which have statistically significant intermolecular and intramolecular compositional differences.
Two successive publications in the journal of Macromolecules, 1989, V22, pages 3851-3866 described blends of isotactic polypropylene and partially atactic polypropylene which purportedly have desirable tensile elongation properties. However, the partially atactic propylene has a broad molecular weight distribution as shown in FIG. 8 of the first publication. The partially atactic polypropylene is also composed of several fractions, which differ in the level of tacticity of the propylene units as shown by the differences in the solubility in different solvents. This is shown by the corresponding physical decomposition of the blend which is separated by extraction with different solvents to yield individual components of uniform solubility characteristics as shown in Table IV of the above publications.
More recently several authors have shown the formation of more refined structures of partially atactic, partially isotactic polypropylene which have elastomeric properties. It is believed that in these components each molecule consists of portions which are isotactic and therefore crystallizable while the other portions of the same polypropylene molecule are atactic and therefore amorphous and not crystallizable. Examples of these propylene homopolymers containing different levels of isotacticity in different portions of the molecule are described in U.S. Pat. No. 5,594,080, in the article in the Journal American Chemical Society (1995), 117, p. 11586, in the article in the Journal American Chemical Society (1997), 119, p. 3635, in the journal article in the Journal of the American Chemical Society (1991), 113, pp. 8569-8570, and in the journal article in the Journal Macromolecules (1995), 28, pp. 3771-3778. These articles describe the copolymer of the present composition but do not describe the compositions obtained in blends with a more crystalline polymer such as isotactic polypropylene, nor its resultant desirable physical properties.
U.S. Pat. Nos. 3,853,969 and 3,378,606, suggest the formation of in situ blends of isotactic polypropylene and xe2x80x9cstereo blockxe2x80x9d copolymers of propylene and another olefin of 2 to 12 carbon atoms, including ethylene and hexene. The copolymers of this invention are necessarily heterogeneous in intermolecular and intramolecular composition distribution. This is demonstrated by the synthesis procedures of these copolymers which involve sequential injection of monomer mixtures of different compositions to synthesize polymeric portions of analogously different compositions. In addition, FIG. 1 of both patents shows that the xe2x80x9cstereo blockxe2x80x9d character, which is intra or intermolecular compositional differences in the context of the description of the present invention, is essential to the benefit of the tensile and elongation properties of the blend of these patents. Moreover, all of these compositions either do not meet all of the desired properties for various applications.
Similar results are purportedly achieved in U.S. Pat. No. 3,262,992 wherein the authors suggest that the addition of a stereoblock copolymer of ethylene and propylene to isotactic polypropylene leads to improved mechanical properties of the blend compared to isotactic polypropylene alone. However, these benefits are described only for the stereoblock copolymers of ethylene and propylene. These copolymers were synthesized by changing the monomer concentrations in the reactor with time. This is shown in examples 1 and 2. The stereoblock character of the polymer is graphically shown in the molecular description (column 2, line 65) and contrasted with the undesirable random copolymer (column 2, line 60). The presence of stereoblock character in these polymers is shown by the high melting point of these polymers and the poor solubility in hydrocarbons at ambient temperature.
Notwithstanding these descriptions of the polymer blends containing isotactic propylene segments it is apparent that useful articles such as elastic fibers have not been constructed from any of these materials. The utility of elastic fibers is that they (a) are soft to the touch, and (b) can recover partly from temporary tensile deformation to less than 100% increase in their original length, this latter affording the wearer of garments based on such fibers more comfort. In addition, there is a need for elastic fibers which are easily processible in conventional thermoplastic plastics fiber equipment using conditions similar to that used for conventional thermoplastic fibers. Further, any or all of the conventional processes used for fiber fabrication should be usable to fabricate the elastic fiber blend. These include but are not limited to the following: continuous filament, bulked continuous filament, staple fibers, melt blown fibers, spun bonded fibers. It is also further desirable to have elastic fibers composed essentially completely of polyolefins such that they are thermally stable, heat resistant, light resistant and generally suitable for thermoplastic applications.
There is a need therefore for soft elastic fibers composed generally essentially completely of polyolefins but having simultaneously a crystalline stereospecific polypropylene component to obtain good tensile strength as well as a crystallizable ethylene-propylene copolymer to provide good elastic recoverability, resistance to elastic flow at a load sustained for specified period as well as a glass transition temperature below that of polypropylene.
Embodiments of our invention include forming soft elastic fibers from predominantly crystallizable, semicrystalline polyolefin copolymers. Further embodiments include improving the aforementioned properties of fibers by blending a generally minor amount of a crystalline polyolefin where the type of crystallinity of the two components are similar, as for instance both will be substantially isotactic or substantially syndiotactic. Isotactic and syndiotactic arrangement of monomers in a polymer are defined in xe2x80x9cPrinciples of Polymerizationxe2x80x9d by G. Odian (3rd Ed) 1991, p. 607 (John Wiley) which is incorporated herein by reference. Substantially pertains to an arrangement of monomer units where greater than 50% of adjacent monomer units have the defined tacticity. Other embodiments of our invention are directed to polyolefins and polyolefin blends where the crystallizable and crystalline components have a stereoregular polypropylene component, especially isotactic polypropylene. A crystalline polymer is one with a heat of fusion, as measured by DSC, to be greater than 50 J/g. A crystallizable polymer is one, with a heat of fusion, as measured by DSC, to be less than 50 J/g. In the semicrystalline, crystallizable copolymer this is achieved with a copolymer of propylene and a C2, C3xe2x80x94C20 alpha-olefin, preferably propylene and at least one other alpha-olefin having 6 or less carbon atoms, and more preferably propylene and ethylene. Improvements in the properties of the semicrystalline, crystallizable copolymer can be obtained by blending it with the crystalline stereoregular polypropylene component, particularly isotactic polypropylene. This crystallizable copolymer is less crystalline than the isotactic crystalline polypropylene. The crystallizable copolymer has a substantially uniform composition distribution, preferably as a result of polymerization with a metallocene catalyst. Composition distribution is a property of copolymers indicating a statistically significant intermolecular or intramolecular difference in the composition of the polymer. Methods for measuring compositional distribution are described later.
We have found that a crystallizable, semicrystalline propylene alpha olefin copolymer, hereinafter referred to as the xe2x80x9cfirst polymer componentxe2x80x9d (FPC) can be used for elastic fibers. The properties of the fiber can be improved by blending an amount of a crystalline propylene polymer, hereinafter referred to as the xe2x80x9csecond polymer componentxe2x80x9d, (SPC). These blends have advantageous processing characteristics while still providing a fiber or fibers having decreased flexural modulus and increased adjusted or normalized load capacity and low values of set and load decay. Soft fibers are those which have a 1% secant modulus less than 25,000 psi in/in, more preferably less than 12,000 psi in/in. It is desirable to have set less than 130% on elongation of the fibers to 400%. The decrease in set and load decay refer to the ability of the elastic fiber to withstand instantaneous and sustained loads, respectively, without deformation.
It is possible to have a third polymeric component which is another crystallizable propylene alpha olefin copolymer indicated as FPC2 in the text below which has crystallinity intermediate between the FPC and the SPC. The FPC2 also has a narrow composition distribution and is made with a metallocene catalyst. The addition of FPC2 leads to a liner morphology of dispersion of the FPC and improvements in some of the properties of the blend of FPC and SPC.
The fibers made from these blends are made by extrusion of the molten polymer through a die as described below, which may be followed by drawing, crimping and/or thermal annealing by any of the other procedures known to the art. Typically, these fibers are between 0.1 to 50 denier in thickness.
According to another embodiment a thermoplastic polymer blend for the preparation of the elastic fiber composition of the invention comprises a SPC and a FPC with added process oil. The SPC comprises isotactic polypropylene, a reactor copolymer or an impact copolymer as described above and is present in an amount of 0% to 95% by weight and more preferably 2% to 70% by weight of the total weight of the blend. The balance of the polymer composition consists of a mixture of the process oil and the FPC and FPC2 if used.
Embodiments of our invention also include a soft, set-resistant, annealed fiber comprising a blend of polyolefins, the blend of polyolefinis being substantially non crosslinked. The blend including a first polymer component (FPC), the FPC has:
i) a composition distribution such that at least 75 weight percent of the polymer is isolated in two adjacent soluble fractions, each of these fractions has a composition difference of no greater than 20% (relative) of the average weight percent ethylene content of the whole first polymer component;
ii) a melting point, as determined by DSC less than 105xc2x0 C.;
iii) a heat of fusion less than 45 J/g;
iv) propylene and an xcex1-olefin present, wherein the xcex1-olefin is present in said FPC from 5-40 weight %, wherein the xcex1-olefin is selected from the group consisting of ethylene and C4-C12 alpha-olefins, propylene making up the balance of the FPC; wherein the FPC is present in the blend from 5-100% by weight.
A second polymer component (SPC), said SPC being a crystalline polymer having:
i) a melting point above 110xc2x0 C.;
ii) a heat of fusion above 60 J/g;
iii) propylene present at least 90 weight %, and an xcex1-olefin present at less than 9 weight %, the total of the propylene and the xcex1-olefin being 100 weight %.
iv) The SPC being present in the blend from 0-95 weight percent, and wherein the fiber exhibits a resistance to set equal to or less than 150% from a tensile deformation of 400%, wherein the blend of polyolefins in said fiber has a flexural modulus equal to or less than 25,000 psi in/in, and may be elongated to 300% in the substantial absence of breakage.
The benefits of embodiments of our invention include improvement in the elastic recovery and the flexural modulus of the elastic fiber made from the blend. These improvements in elastic recovery are most apparent for 400% tensile extension of the fiber. Historically, the examples of the prior art have not been able to duplicate either the extensibility of the fiber to 400% or any significant portion of the elastic recovery of the polymer fiber.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims.